Ni2+ produced from anode gains electrons at cathode to become Ni atoms becoming nickel. Hence, the nickel is deposited here and the electrode grows.

Ni2+ (aq) + 2e- --> Ni (s)

Overall Change

There is no change in solution contents while iron object receives nickel deposit.

Uses of Electroplating

Creation of Electric Cells by Electrolysis

A Simple cell or an Electric cell is a device that converts chemical energy into electrical energy, and it consists of 2 electrodes made of 2 metals of different reactivity.

In a simple cell, the MORE REACTIVE metal/electrode is ALWAYS designated the NEGATIVE electrode

The anode (negative electrode) is made of more reactive metal. This is because they have more tendency of losing electrons.

The cathode (positive electrode) is made of less reactive metal.

The further apart the metals in the reactivity series, the higher the voltage created.

The electrons in a simple cell will ALWAYS flow from the NEGATIVE electrode (made of the MORE reactive metal) to the POSITIVE electrode.

Eg. A simple electric cell using zinc and copper

Observation: bubbles of hydrogen gas appear at the copper rod.

Explanation: Zinc is more reactive than copper. Thus, it is more electropositive than copper, meaning that zinc loses electrons more easily than copper. As a result, oxidation occurs at the zinc rod (the anode) and zinc metal loses electrons to become zinc ions, that is, Zn (s) - 2e- -->Zn2+(aq)

The electrons then flow from the zinc rod to the copper rod through the external circuit. At the copper rod, reduction occurs - the hydrogen ions in solution accept these electrons to form hydrogen gas;

2H+(aq) + 2e- --> H2(g)

This explains why bubbles of gas are produced at the copper rod when the two rods are connected by a wire.

The magnitude of the voltage (potential difference) is related to the positions of the two metals in the reactivity series. The further apart the two metals, the larger will be the potential difference (voltage) produced.

Electrolytic cell vs Electrochemical cell

Electrolytic cell

Electrochemical cell

Converts chemical energy into electrical energy

Converts electrical energy into chemical energy

Redox reaction is spontaneous and is responsible for the production of electrical energy

Redox reaction is not spontaneous and electrical energy has to be supplied to initiate the reaction

The two half-cells are set up in different containers which are connected through a salt bridge or porous partition

Both the electrodes are placed in the same container in a solution of molten electrolyte.

The anode is the negative electrode while the cathode is the positive electrode. The reaction at the anode is oxidation and the reaction at the cathode is reduction.

The anode is the positive electrode while the cathode is the negative electrode. The reaction at the anode is oxidation and the reaction at the cathode is reduction.

Electrons are supplied by the species getting oxidized. They move from anode to the cathode in the external circuit.

An external battery supplies the electrons, which enter through the cathode and come out through the anode.

Factors affecting electrolysis

Concentration

Type of electrode

Concentration

If the concentration of a particular ion is high, then this can alter the preferential discharge

If dilute hydrochloric acid is electrolysed, hydrogen gas is given off at the cathode and oxygen gas at the anode. However, when concentrated hydrochloric acid is electrolysed, hydrogen gas is still given off at the cathode, but chlorine gas is given off at the anode.

This is because although the chloride ion is harder to discharge than the hydroxide ion, its high concentration makes it more likely to be discharged.

Type of electrode

Eg. electrolysis of aqueous copper(II) sulphate solution

Use Carbon Electrodes:

Carbon electrodes are inert and so do not affect the electrolysis

At the anode, we have a choice of sulphate or hydroxide ions, and hydroxide ions are easier to discharge so oxygen gas is given at the anode

4OH- (aq) + O2 (g) ---> O2 (g) + 2H2O (l) + 4e-

At the cathode, we have a choice of copper or hydrogen ions. Copper ions are easier to discharge so we will see a pinkdeposit of copper metal on the carbon electrode

MCQ Questions

2. When a dilute salt water is electrolysed, a colorless gas is given off at the anode. The gas isa. hydrogenb. steamc. oxygend. chlorine

3.
A solution of copper(II) sulphate is electrolysed, using carbon
electrodes. The pinkish deposit which forms on one of the electrodes isa. copperb. copper(I) oxidec. copper(II) oxided. copper(III) sulphide

4. A solution of copper(II) sulphate is electrolysed, using copper electrodes. Which of the following would happen?a. the anode loses weightb. the cathode loses weightc. the solution darkens in colord. the solution lightens in color

5. An electrolyte is alwaysa. an acid or alkalib. an aqueous solutionc. a liquidd. a molten solid

10. Chlorine is manufactured commercially by the electrolysis of aqueous sodium chloride (brine). Which other important products are made in the process?a. hydrochloric acid and hydrogenb. hydrogen and sodiumc. hydrogen and sodium hydroxided. sodium and sodium hydroxide

11. An electric current is passed through aqueous potassium sulphate, K2SO4.

What is formed at the cathode (negative electrode)?a. hydrogenb. oxygenc. potassiumd. sulphur

13. Which element is liberated at a carbon cathode when aqueous sodium chloride is electrolysed?a. chlorineb. hydrogenc. oxygend. sodium

14. Which change always takes place when aqueous copper(II) sulphate is electrolysed?a. copper is deposited at the negative electrodeb. oxygen is evolved at the positive electrodec. sulphate ions move towards the negative electroded. the color of the solution fades

15. Which element is liberated at the cathode by the electrolysis of an aqueous solution containing its ions?a. bromineb. chlorinec. hydrogend. oxygen

16. Aqueous copper(II) sulphate is electrolysed using copper electrodes. Which observations will be made?

at anode (positive)

at cathode (negative)

a

anode dissolves

pink solid forms

b

anode dissolves

pink solid forms

c

color gas forms

color gas forms

d

color gas forms

pink solid forms

17. Why is cryolite, Na3AlF6, used in the extraction of aluminium from aluminium oxide?a. to dissolve aluminium oxideb. to prevent the anodes from burning awayc. to prevent the oxidation of the aluminiumd. to remove impurities from the aluminium oxide

18. When sodium chloride was electrolysed, sodium was produced at the negative electrode. In which form was the sodium chloride during the electrolysis?

a. concentrated aqueous solution

b. dilute aqueous solution

c. molten

d. solid

19. In which instance is there no change in the concentration of the solution during electrolysis?

a. concentrated sodium chloride solution between carbon electrodes

b. copper(II) sulfate solution between copper electrodes

c. copper(II) sulfate solution between platinum electrodes

d. dilute sodium chloride solution between platinum electrodes

20. An example of a weak electrolyte is

a. alcohol

b. salt solution

c. sugar solution

d. ammonia solution

21. Electroplating iron with zinc is called galvanising. The reaction at the cathode is shown by the equation

a. Fe (s) ---> Fe2+ (aq) + 2e-

b. Fe2+ (aq) + 2e- ---> Fe (s)

c. Zn (s) ---> Zn2+ (aq) + 2e-

d. Zn2+ (aq) + 2e- ---> Zn (s)

22. The circuit shown below was set up, with brass as the anode.

Which electrode reactions will occur on closing the switch?

Anode reaction Cathode reaction

a.Copper dissolves preferentially. Copper is deposited.

b. Copper dissolves preferentially. Hydrogen is evolved.

c. Zinc dissolves preferentially. Hydrogen is evolved.

d. Zinc and copper both dissolve. Copper is deposited.

23. During the electrolysis of concentrated sodium chloride in a cell, chlorine, hydrogen, and sodium hydroxide are produced. What is the molar ratio of these products?

Chlorine Hydrogen Sodium hydroxide

a. 1 1 1

b. 2 1 2

c. 2 1 1

d. 2 2 1

Answers

1. b2. c3. a4. a5. c6. c7. a8. b9. a10. c11. a (H+ and K+ ions in the electrolyte migrate to the cathode. H+ are preferentially discharged to form hydrogen gas because it is lower down in the electrochemical series than K+ ions)12. b (the negative chloride ions will migrate to the anode and become oxidised at the anode to form chlorine gas)13. b (the ions attracted to the cathode are H+ and Na+ ions. H+ is preferentially discharged to form hydrogen gas)14. b15. c16. b17. a

18. c

19. b

20. d

21. d

22. c

23. b

Structured Question Worked Solutions

1. Dilute sulphuric acid will conduct an electric current.a. Give the formulae of all of the ions present in dilute sulphuric acid

b. Name the gaseous products which you would expect to be formed during the electrolysis of aqueous potassium sulphate using inert electrodesat the anode:_____at the cathode:______

c. Name a metal which is used to electroplatei. bicycle handlebarsii. teaspoon

d. Explain why a metal such as aluminium can conduct an electric current but a non-metal such as sulphur cannot conduct a current

Solution

a. H+, OH-, SO42-

b. cathode: Hydrogenanode: Oxygen

ci. chromiumcii. silver

d. Aluminium consists of positively charged particles in a sea of electrons. The electrons are able to move freely and thus electricity can flow. In sulphur, the atomic arrangement is fixed, so there is no movement of electrons. When electricity is passed through sulphur, electricity will not be able to flow.

2a. When concentrated aqueous sodium chloride is electrolysed using graphite electrodes, hydrogen is collected at the cathode and chlorine at the anode.

When concentrated aqueous sodium chloride is electrolysed using iron electrodes, hydrogen is again collected at the cathode but much less chlorine is collected at the anode.

i. Give the equations for the electrode reactions by which hydrogen and chlorine are formed

ii. Explain why much less chlorine is collected when iron electrodes are used.

iii. Name the product, other than hydrogen and chlorine, which is manufactured by the electrolysis of concentrated aqueous sodium chloride. Give a major use of this product

b. Why is the electrolysis of concentrated hydrochloric acid not used for the manufacture of chlorine?

Answers

ai. 2H+ (aq) + 2e- -----> H2 (g)2Cl- (aq) -----> Cl2 (g) + 2e-

ii. When iron anode is used, some oxygen gas is produced at the same time. Some of the electrical energy is used to liberate oxygen. So less Cl2 is produced.

iii. Sodium hydroxide. It is used to manufacture soap

b. because concentrated HCl is not a cheaply and readily available raw material. It is also a volatile acid. A lot of HCl gas will be emitted.

3.

The diagram shows the electrolytic cell used to produce aluminium. The electrolyte contains aluminium oxide and cryolite (sodium aluminium fluoride) and is molten at about 800oC. The electrodes are made from graphite.

a. Why is a mixture of cryolite and aluminium oxide, and not pure aluminium oxide, used as the electrolyte?

b. Write the equations for the reactions occurring ati. the positive electrodeii. the negative electrode

c. Explain why the graphite anodes need to be replaced at regular intervals

d. Calculate the maxiumum mass of aluminium that can be made from 408 tonnes of aluminium oxide.

ei. Aluminium foil is used to make food containers because it does not corrode easily. Explain why aluminium does not corrode easily.

eii. Give a use of aluminium, other than for food containers, together with the physical property that makes aluminium suitable for that use.

eiii. Give a further use of aluminium, that makes aluminium suitable for that use. (excluding that from ei and eii)

Solution

a. Aluminium oxide has a very high melting point. With the addition of cryolite, the melting point is greatly reduced, making it more economical.

bi. 2O2- (l) ---> O2 (g) + 4e-bii. Al3+ (l) + 3e- ---> Al (s)

c. The oxygen produced at the graphite anode oxidises the graphite to CO2

ei. Aluminium forms aluminium oxide in the presence of air. This oxide is insoluble and resistant to corrosion so it forms a protective coating for aluminium.

eii. It is used in making cooking utensils since it has very good conductivity, in addition to its good appearance and resistance to corrosion.

eiii. it is a component in several alloys used in aircraft construction. Its favorable use is due to its low density and high tensile strength.

4. Complete the table by naming the products formed when the following liquids are electrolysed using inert electrodes

liquid

product formed at cathode

product formed at anode

dilute sulphuric acid

oxygen

molten calcium bromide

concentrated aqueous sodium chloride

Solution

liquid

product formed at cathode

product formed at anode

dilute sulphuric acid

hydrogen

oxygen

molten calcium bromide

calcium

bromine

concentrated aqueous sodium chloride

hydrogen

chlorine

5. Aqueous copper(II) sulphate was electrolysed in two cells using different electrodes as shown below.

a. Write ionic equations, with state symbols, for the reactions which take place at the anode in each cell.b. Describe one change that you would see happen in both cells.c. Describe one change that you would see happen in Cell 1 but not in Cell 2.

Solution

a. Anode reaction cell 1: 4OH- (aq) --> O2 (g) + 2H2O (l) + 4e-

Anode reaction cell 2: Cu (s) --> Cu2+ (aq) + 2e-

b. The size of the cathode increases as copper metal is plated onto the cathode in both cells.

Cu2+ (aq) + 2e- --> Cu (s)

c. The blue colour of the electrolyte in cell 1 fades when more and more Cu2+ ions are reduced to copper metal and plated onto the cathode as a pink deposit.

b. Draw an arrow on the diagram to show the direction of the flow of electrons in the wire.

c. The voltage of the cell was measured when the following metals were used as electrode 2.copperironleadzinc

Complete the table by entering the metals in the correct order.

meter reading/V

Metal

1.10

0.78

0.21

0.00

d. When metal M was used as electrode 2, it produced a higher voltage than zinc. Suggest a name for metal M.

Solution

a. Zn (s) + Cu2+ (aq) --> Zn2+ (aq) + Cu (s)

Note: zinc, being a more reactive metal, displaces copper ions out of solution as copper metals

b. arrow direction from electrode 2 to electrode 1

Note: Zinc, being a more reactive metal, loses electrons more easily when connected to a metal of lower reactivity, in this case copper.

c.

meter reading/V

Metal

1.10

zinc

0.78

iron

0.21

lead

0.00

copper

d. Magnesium

7. Electroplating can be used to coat nickel with a thin coating of silver.

a. Draw a labelled diagram of an apparatus that can be used to electroplate silver onto nickel.

b. Write equations, with state symbols, for the reactions at the anode and cathode.

c. Solutions of two salts, A and B, were electrolysed using carbon electrodes. The following products were collected.

Salt

Products

A

oxygen and hydrogen

B

chlorine and hydrogen

i. Suggest the names of the two salts, A and B.

ii. Describe tests to confirm the identities of the three gases collected.

Solution

a.

b. reaction at anode: Ag (s) --> Ag+ (aq) + e-

ci. Salt A: sodium sulphate

Salt B: sodium chloride

cii. Collect samples of each gas using test tubes. To test for oxygen, insert a glowing splint into the test tube of gas. The gas that relights the glowing splint is oxygen. To the remaining samples, place a lighted splint at the mouth of each test tube. The gas that extinguishes the lighted splint with a "pop" sound is hydrogen. To identity chlorine, place a piece of moist blue litmus at the mouth of test tube of gas. The litmus turns red and bleaches.

8. One important use of a gas Y is to sterilize swimming pool water. The gas is prepared in the laboratory by the electrolysis of a solution. A student tried to prepare gas Y by the electrolysis of a very dilute sodium chloride solution as shown below. Contrary to the student's expectation, a colorless gas, instead of gas X, was liberated at the anode.

a. What is the colorless gas liberated?b. Suggest a chemical test for the colorless gasc. The experiment was then modified to prepare gas Y.i. Suggest how the experiment could be modified. Explain your answer.ii. Suggest the solution left after the electrolysisiii. Suggest one common use of the solution left.

Solution

8a. oxygen

8b. The gas relights a glowing splint.

8ci. Use concentrated sodium chloride solution instead of a very dilute sodium chloride solution. Concentration of chloride ions in the solution would be much greater than that of hydroxide ions. Therefore, chloride ions would be preferentially discharged to form chlorine gas.

a. Describe and explain what would happen at the two carbon electrodesb. Write half-equations for the reactions at carbon exlectrodes X and Y.c. What will be the charge in the electrolyte as electrolysis proceeds for some time? Explain your answer.d. Explain what could happen to the copper(II) sulphate solution if copper electrodes are used in the above experiment.

Solution

9a. At carbon electrode X:The sulphate ions and hydroxide ions migrate to electrode X. A hydroxide ion is a stronger reducing agent than a sulphate ion. So hydroxide ions are preferentially discharged.

At carbon electrode Y:The copper(II) ions and hydrogen ions migrate to electrode Y. A copper(II) ion is a stronger oxidizing agent than a hydrogen ion. So copper(II) ions are preferentially discharged to form a deposit of copper on electrode Y.

9b. electrode X:4OH- (aq) --> O2 (g) + 2H2O (l) + 4e-

electrode Y:Cu2+ (aq) + 2e- --> Cu (s)

9c. The solution becomes sulphuric acid because copper(II) ions and hydroxide ions are consumed in the electrolysis. Hydrogen ions and sulphate ions remain in the solution.

9d. The net effect is the transfer of copper from electrode X to electrode Y. The dilute copper(II) sulphate solution remains the same.

10. Tuning knobs on radios are often made of plastics plated with metal coatings. The plastic knobs are first coated with copper and then electroplated with nickel. The electroplating can be conducted using the following setup.

a. Explain the term 'electroplating'b. Why is the plastic knob first coated with copper before electroplating?c. Explain why nickel(II) sulphate solution can conduct electricityd. Which is the anode, the nickel electrode or the copper-coated knob?e. Write an ionic half-equation for the reaction at the copper-coated knobf. Explain why it is better to use a nickel electrode than a carbon electrode in the above processg. In a nickel-plating factory, the waste water is treated with sodium hydroxide solution to remove nickel(II) ions before discharge. Suggest 2 reasons why it is necessary to remove nickel(II) ions from the waste water before discharge

Solution

10a. It is the coating of an object with a thin layer of a metal by electrolysis10b. to make the knob conduct electricity10c. the solution contains mobile ions10d. the nickel electrode10e. Ni2+ (aq) + 2e- --> Ni (s)10f. the concentration of nickel(II) ions in the electrolyte can be maintained10g. - to recover the nickel metal - nickel(II) ions are harmful to marine lives. Humans may get poisoned by eating contaminated seafood

11. The following circuit is set up. Electrodes A and B are made of carbon while electrode C and D are made of copper.

a. What are the functions of the ammeter and rheostat respectively?b. Explain why no current flows when potassium iodide is in solid state, but a current flows when water is added.ci. What would be observed at electrode A and B respectively?cii. Write ionic half-equations for the reactions at electrodes A and Bdi. What would be observed at electrodes C and D respectively?dii. Write ionic half-equations for the reactions at electrodes C and Ddiii. Would you expect any color change in the dilute copper(II) sulphate solution during the process? Explain your answer.

Solution

11a. The ammeter is an instrument used to measure the electric current passing through the circuit. The rheostat is used to vary the resistance in the circuit and regulate the current.

11b. In solid state, the ions in potassium iodide are held together by strong attraction. They are not free to move. So solid potassium iodide does not conduct electricity. When water is added to the compound, the compound dissolves in the water and the ions become mobile and a current can then flow through the solution.

11ci. A brown color develops around electrode A. A colorless gas is given off from electrode B.

11diii. The blue color of the dilute copper(II) sulphate solution does not change because the concentration of copper(II) ions in the solution remains the same.

12. A student used the following set-up for passing electricity through some solutions.

The results are shown below:

Experiment

Solution

Observations

1

sugar solution

zero ammeter reading

2

dilute sulphuric acid

gas bubbles given off from both electrodes

3

dilute sodium iodide solution

?

4

dilute silver nitrate solution

gas bubbles given off from one electrode and silvery solid deposited on the other electrode

a. Why is zero ammeter reading recorded in Experiment 1?b. For Experiment 2, i. name the gas liberated at electrodes X and Y respectivelyii. write ionic half-equations for the reactions at the electrodes

c. For Experiment 3,i. what substance would you expect to form at electrode X. Explain briefly.ii. what substance would you expect to form at electrode Y. Explain briefly.

d. For Experiment 4,i. write ionic half-equations for the reactions at the electrodesii. state the change in the solution after electricity has been passed through for some time.

Solution

12a. Sugar solution is not a conductor of electricity

12bi. X: oxygen; Y: hydrogen

12bii.at electrode X:4OH- (aq) --> O2 (g) + 2H2O (l) + 4e-

at electrode Y:2H+ (aq) + 2e- --> H2 (g)

12ci. The concentration of iodide ions in the solution is much greater than that of hydroxide ions. Iodide ions are preferentially discharged to form iodine.

12cii. A hydrogen ion is a stronger oxidizing agent than a sodium ion. So hydrogen ions are preferentially discharged to form hydrogen gas.

12dii. Silver ions and hydroxide ions are consumed in the electrolysis. Hydrogen ions and nitrate ions remain in the solution. The solution eventually becomes nitric acid solution.13. When the circuit in the set-up shown below is closed, the acidified potassium permanganate solution loses its color gradually.

a. Write a half equation for the reaction that occurs in the acidified potassium permanganate solution. Explain whether the permanganate ion is oxidized or reduced.

b. What would be observed in the iron(II) sulphate solution after some time. Write a half equation for the reaction that would occur.

c. Identify the direction of electron flow in the external circuit.

d. Write an ionic equation for the reaction that occurs when acidified potassium permanganate solution and iron(II) sulphate solution are mixed together.

ei. Give the function of the salt bridge set upeii. Explain whether a sodium sulphite solution can be used instead of a potassium nitrate solution in the salt bridge.

As an alternative to iron(II) sulphate and acidified potassium permanganate solution, potassium iodide solution and iron(III) sulphate solution may be used on the left hand side and the right hand side respectively.

fi. What would be observed in the potassium iodide solution after some time? Write a half equation for the reaction that would occur.fii. What would be observed in the iron(III) sulphate solution after some time? Write a half equation for the reaction that would occur.

ai. which substance is the positive electrode?aii. write an ionic half equation for the reaction at the positive electrode

bi. which substance is the negative electrode?bii. write an ionic half equation for the reaction at the negative electrode

c. explain the function of manganese(IV) oxide

d. the voltage of the cell drops if current is drawn from the cell rapidly. Explain briefly.

e. explain why the zinc case of a used cell is thinner than that of a new cell

f. explain why disposal of 'flat' cells present a pollution problem

g. explain why zinc-carbon cells should be removed from electric appliances when not in use for a long period

Solution14ai. carbon rod

14aii. 2NH4+ (aq) + 2e- --> 2NH3 (aq) + H2 (g)

14bi. zinc case

14bii. Zn (s) --> Zn2+ (aq) + 2e-

14c. Hydrogen is produced and collected on the surface of the positive electrode. Since hydrogen is a poor conductor of electricity, the accumulation of hydrogen at the positive electrode may hinder further reactions and decrease the current of the cell. Manganese(IV) oxide, an oxidizing agent, is used to remove the hydrogen.

14d. If a current is drawn from the cell rapidly, the gaseous product cannot be removed fast enough. The voltage drops as a result.

14f. The materials inside the cells do not decompose even after a long time. These materials may combine with other compounds and form harmful substances which pollute the environment.

14g. There is a slow direct reaction between the zinc electrons and ammonium ions. After some time, the zinc case becomes too thin and the paste leaks out. This may cause damage to electric appliances.